1. Field of the Invention
The present invention refers to a high-pressure discharge lamp to be utilized e.g., for general illumination or for projection display, a method for manufacturing a discharge lamp body for high-pressure discharge lamps, and a method for manufacturing a hollow tube body.
2. Description of the Prior Art
Thus far, for metal halide discharge lamps, quartz glass components (comprising nearly 100% SiO.sub.2) has often been used.
However, defects in quartz glass material are mentioned in that quartz glass becomes likely to react with the high-pressure gas enclosed in a lamp when the duration of lamp lighting increases, thereby inevitably decreasing the optical transmissivity, that a marked low thermal conductivity (approx. 0.9 W/mK) hinders the distribution of heat from becoming uniform, and the like.
Furthermore, there has occurred also a problem that the internal heat convection stimulated by the above nonuniform temperature distribution results in a large curvature of discharge arc.
Thus, a countermeasure is also considered that a protective layer comprising a monolayer or multi-layers aluminum oxide coating, tantalum oxide coating or others is provided on the interior of a quartz glass discharge tube body (e.g., U.S. Pat. No. 5,270,615 Specification).
However, as a defect due to such a countermeasure in conventional discharge tube bodies, it is mentioned that the corrosion resistance of an oxide coating at high temperature is not high enough for practical use.
That is, since reaction of rare earth metal halide enclosed in a lamp with the oxide coating is perceived in a state of high temperature near to 1000.degree. C. during lamp lighting, it can be said in the conventional countermeasure mentioned above that the preventive effect on devitrification remains still insufficient.
Also, because an oxide coating was used as a protective coating, there was an insufficient point that no effect of thermally uniformizing a discharge tube body cannot be obtained.
On the other hand, as another countermeasure, there has been made an attempt to obtain effects of preventing the devitrification due to a high corrosion resistance, uniformizing the temperature distribution in a discharge tube body due to a high thermal conductivity and further improving the heat load characteristic by using a ceramic (Al.sub.2 O.sub.3, AlN, YAG, spinels or the like) discharge tube body (e.g., Japanese Patent Publication No. 87938/1993).
However, the ceramic discharge tube body mentioned above has defects in that corrosion in the sealing portion between a ceramic tube body and the end face cannot be ignored, that its characteristic deviates from that of an ideal point light source as a result of a fall in straight light transmissivity due to intergranular reflection in a ceramic sinter and the like, so that it is kept from being put into practical use.
Also, the ceramic discharge tube body mentioned above generally arouses a discontent that the cost is high and a complicated manufacturing process is needed in comparison with a quartz glass tube body.
For solving the above conventional problems, the present invention has an object in achieving a high-pressure discharge lamp capable of preventing the devitrification more efficiently and having a longer useful life than former by using an oxynitride coating indicative of higher durability than that of a conventional oxide coating as the inside wall of a discharge tube body.
Meanwhile, the linear expansion coefficient of quartz glass is characteristically small (0.54 ppm/.degree.C.). Even if aluminum oxide (7-8 ppm/.degree.C.) or other metal oxides having a large linear expansion coefficient is formed directly on quartz glass as a corrosion-resistant coating, the inside wall coating comes to crack or peel off under action of dynamic mechanical stress generated when a high temperature (approx. 1000.degree. C. at the maximum) during operation of a lamp and a room temperature during extinction are repeated and consequently a substantially durable structure has not yet implemented at present from the practical standpoint.
The aforesaid U.S. Pat. No. 5,270,615 intends to solve the above problems by using an oxide coating having a thermal expansion coefficient ranging from 1 to 4 ppm/.degree.C. as the under coating, but this is also still insufficient. Thus, it is another object of the present invention to provide a novel coating structure having a greater durability in practical use with account paid to a substantial linear expansion coefficient in each constituent layer of the protective layer.